1. Field of the Invention
The present invention relates to a method of longitudinal rolling of seamless pipes in a continuous rolling procedure over an internal tool in a multiple-stand rolling train, wherein the rolls of successive rolling stands are arranged offset relative to each other.
2. Description of the Related Art
In known rolling procedures for the manufacture of seamless steel pipes, a distinction is made between transverse rolling procedures and longitudinal rolling procedures. In the transverse rolling procedure, the shape of the pass of the deforming rolls does not have to be adapted to the cross section of the pipe. On the other hand, in the longitudinal rolling procedure, an essentially round or closed pass shape is required.
In addition, in longitudinal rolling procedures with internal tool of the type to which the present invention is directed, a distinction is made between single-stand rolling mills, such as, the Stiefel-type piercing rolling mill, and multiple-stand trains, such as, the known continuous pipe rolling train with a rolling rod or mandrel which travels through freely or with a rolling mandrel which is pulled along in a controlled manner. The continuous rolling trains have the disadvantage that they require a relatively large number of rolling stands which constitute a substantial investment. While 20 years ago eight rolling stands were still required, the number of stands was reduced during the following years. It has already been proposed to construct rolling trains with five rolling stands (speech by Palma, Tube 91, Chicago from Jun, 17 to Jun, 19, 1991 "NEW TENDS IN SEAMLESS TUBE MAKING"). This publication also contains proposals to reduce the number of stands to 4 or 3. The publication further mentions that when fewer than 5 stands are used, only a simple pipe length of approximately 15 meters can be achieved. On the other hand, when using rolling trains with 5 or more stands, double pipe lengths of up to approximately 30 meters can be achieved in pipes having thin walls and pipes having normal walls. Pipes having normal walls are considered in the art to be pipes having dimensions as defined in DIN 2448. Pipes having thin walls have a wall thickness of 90% of normal walls.
It is a prerequisite that the inclined piercing mill before the rolling train produces pierced shells or hollow blanks which are sufficiently long. Modem inclined piercing mills produce hollow blank lengths of 12 meters and more.